1. Field of the Invention
The present invention relates to a thermal head mounted on a thermal printer and the like, a method for manufacturing the same, and a method for adjusting a dot aspect ratio of a thermal head.
2. Description of the Related Art
In general, a thermal head includes a plurality of heating element portions which generate heat by energization, electrode layers to energize the plurality of heating element portions, and a protective layer to protect the plurality of heating element portions and part of the electrode layers, on a heat dissipating substrate provided with a heat storage layer. The heating element portion generating heat is pressed against an ink ribbon and a printing substrate wound around a platen roller and, thereby, the printing operation is performed. In such a known thermal head, each heating element portion to produce one printing dot is formed into the shape of a rectangle. But, it is desirable that the aspect ratio (length-to-width ratio) L/W of one printing dot is brought close to 1 (square pixel) as much as possible in order that the printing can be performed with high precision in both the vertical direction and the horizontal direction, as disclosed in Japanese Unexamined Patent Application Publication No. 5-50630.
However, when the dot aspect ratio L/W is brought close to 1, the amount of etching tends to vary in a photolithography step to form a plurality of heating element portions, and there is a problem in that variations in resistance value (dot resistance value) of the plurality of heating element portions are increased. Variations in dot resistance value must be minimized since variations in dot resistance value cause variations in printing concentration during printing. If variations in dot resistance value exceed a specific level, no product of satisfactory quality is attained and, therefore, the yield is decreased. When the dot aspect ratio L/W is brought close to 1, the area thereof becomes smaller than the area of a known heating element portion. Consequently, the dot resistance value must be increased, and a detriment occurs in that each heating element portion must be formed from a resistance material having a high resistivity.